Before you mentioned an issue with preen gland. This is often linked with a poor diet , like an all seed diet. Lacking vitamins found in fresh veggies, sprouts, some fruits, or pellets. Especially linked with low vitamin A.
It can also get infected or impacted with or without infection. Still often linked with diet initially
Pictures really would help. Please don't feel bad if your bird is missing feathers from plucking. Plucking is complex , can be health or behavioral. And many member have birds that pluck